388 THE BIOLOGY OF INSECTS 



whose females lay their eggs in the galls, so that the grubs 

 may devour the inhabitants whether hosts or inquiUnes. 



Gall-formation on plants is brought about as a response 

 to the presence of many other insects besides Cynipidae . 

 The sawflies (Tenthredinidae), also belonging to the 

 Hymenoptera, have a number of gall- dwelling species ; the 

 succulent fruit-like growths often seen beneath the leaves of 

 willows harbour caterpillars of Pontania, a genus of the 

 Nematine section of the family. Here the abnormal plant- 

 growth seems to result from the egg-laying. The gall- 

 midges (Cecidomyidae), a family of small Diptera, comprise 

 hundreds of species whose larvae live in characteristic galls 

 formed on various kinds of plants. On willows, for 

 example, the little pinkish grubs of Rhabdophaga heterohia 

 may be found crowded among the small rosette-like hairy 

 leaves of the arrested shoots, while the larvae of the allied 

 R. saliciperda feed in small oval chambers in the outer 

 layers of the wood ; when transformation is completed their 

 empty white pupal cuticles project from the surface of the 

 bark. In female gall-midges the hinder " telescoped " 

 segments of the abdomen are very elongate, so that the 

 slender tip can be inserted between young leaves or else- 

 where that may be suitable for egg-laying. The Pear- 

 Midge {Coniarinia pyrivora), for example, pushes her long, 

 tapering tail-region into a blossom-bud in early spring and 

 the grubs hatched from her eggs feed in the young fruitlet. 

 In this case there is no true gall-formation ; the little 

 infested pears become shrivelled and stunted, and usually 

 fall to the ground early in the season. The full-grown 

 pear-midge grubs have the habit of bending their bodies 

 sharply and then suddenly straightening them ; thus they 

 are enabled to leap out of the pear, whether it be fallen to 

 the ground or still on the tree, and to reach the surface of 

 the soil into which they burrow for pupation. Here, again, 

 the form and behaviour of the insect, both in the larval and 

 winged stages, ensures its adaptation to the plant on which 

 it is dependent. 



An interesting development in the relations between 



